Bluetooth: GATT Subscribe does not detect duplicate if new parameters are used.

[paulgavrikov]

Describe the bug
Similar to #15098. It is possible to issue a new subscribe command to previously subscribed handles by using a different subscribe param struct.

Expected behavior
Iterare over existing subscriptions and compare handles.

[Vudentz]

We do support multiple subscription to the same CCC, it does actually iterate to check that:

https://github.com/zephyrproject-rtos/zephyr/blob/master/subsys/bluetooth/host/gatt.c#L2733

Im closing it as it is working as intended, if you want to add an API to query existing subscription that is a different API, though Id consider a bug if an application cannot really check its own subscription since the parameters are owned by it.